AP Chemistry
Hess’ Law
Thermo Unit
Objectives:
I can…
 Use Hess’ Law to determine the heat of a reaction.
Hess’ Law
 If a reaction is carried out in a series of steps, ΔH for the reaction will be equal to the sum of the enthalpy
changes for the individual steps.
 Rearranging stepwise reactions in order to find the overall enthalpy of a reaction
Examples:
Show All Work on a Separate Sheet of Paper!!
1. Calculate the ΔHrxn of N2 (g) + 2O2 (g)  2NO2 (g) based on the following reactions:
A.) N2 (g) + O2 (g)  2NO (g)
B.) 2NO (g) + O2 (g)  2NO2 (g)
ΔH = +180 kJ
ΔH = -112 kJ
2. Calculate the ΔHrxn of C(graphite)  C(diamond) based on the following reactions:
A.) C(graphite) + O2 (g)  CO2 (g)
B.) C(diamond) + O2 (g)  CO2 (g)
ΔH =-394 kJ
ΔH = -396 kJ
3. Calculate the ΔHrxn of 2C2H6 (g) + 7O2 (g)  4CO2 (g) + 6H2O (g) based on the following reactions:
A.) 2 C (s) + 3H2 (g)  C2H6 (g)
B.) C (s) + O2 (g)  CO2 (g)
C.) H2 (g) + ½ O2 (g)  H2O (g)
ΔH = -84.7 kJ
ΔH = -393.5 kJ
ΔH = -242 kJ
4. Calculate the ΔHrxn of NH4Cl  NH3 + HCl based on the following reactions:
A.) N2 + 3 H2  2 NH3
B.) H2 + Cl2  2 HCl
C.) N2 + 4 H2 + Cl2  2 NH4Cl
ΔH = -91.2 kJ
ΔH = -184.6 kJ
ΔH = -618.8 kJ
5. Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
2CO2(g) + H2O(g) → C 2H2(g) + 5/2O2(g)
A.) C2H2(g) + 2H2(g)
→ C2H6(g)
B.) H2O(g)
→ H2(g) + 1/2O2 (g)
C.) C2H6(g) + 7/2O2(g) → 2CO2(g) + 3H2O(g)
6. Calculate the ΔHrxn of C (s) + H2O (g)  CO (g)+ H2 (g)
A.) C (S) + ½ O2 (g)  CO (g)
B.) C (s) + O2 (g)
 CO2 (g)
C.) CO (g) + ½ O2 (g)  CO2 (g)
D.) H2 (g) + ½ O2 (g)  H2O (g)
ΔH =-94.5 kJ
ΔH =+71.2 kJ
ΔH =-283 kJ
based on the following reactions:
ΔH
ΔH
ΔH
ΔH
=
=
=
=
-110.5
-393.5
-283.0
-242.0
kJ
kJ
kJ
kJ
7. Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
NO (g) + O (g)  NO2 (g)
A.)
B.)
C.)
8.
NO
(g)
2 O3 (g)  3 O2 (g)
O2 (g)  2 O (g)
+ O3 (g)  NO2 (g) + O2
ΔH = - 427 kJ
ΔH = + 495 kJ
ΔH = - 199 kJ
(g)
Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
CO2 (g) → C (s) + O2 (g)
A.)
H2O (l) → H2 (g) + ½ O2 (g)
B.)
C2H6 (g) → 2 C (s) + 3 H 2 (g)
C.) 2 CO2 (g) + 3 H2O (l) → C 2H6 (g) + 7/2 O2(g)
ΔH = + 286 kJ
ΔH = - 190.6 kJ
ΔH = + 3511.1 kJ
9. Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
½ H2 (g) + ½ Cl2 (g) → HCl (g)
A.)
B.)
C.)
CH2Cl2 (l)
COCl2 (g) + H2O (l) → CH2Cl2 (l) + O2 (g)
2 HCl (g) + ½ O2 (g) → H2O (l) + Cl2 (g)
+ H2 (g) + 3/2 O2 (g) → COCl2 (g) + 2 H2O
ΔH = + 47.5 kJ
ΔH = +105 kJ
ΔH = -402.5 kJ
(l)
10. Calculate the ΔHrxn of Fe3O4 (s) + CO (g)  3 FeO (s) + CO2 (g)
based on the following reactions:
A.) 2 Fe (s) + 3 CO2 (g)  Fe2O3 (s) + 3 CO (g)
B.) 2 Fe3O4 (s) + CO2 (g)  3 Fe2O3 (s) + CO (g)
ΔH = - 23 kJ
ΔH = - 39 kJ
C.) FeO (s) + CO (g)  Fe (s) + CO2 (g)
ΔH = + 18 kJ
11. Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
HCl (g) + NaNO2 (s) → HNO2 (l) + NaCl (s)
A.)
B.)
C.)
D.)
NO
(g)
2 NaCl (s) + H2O (l) → 2 HCl (g) + Na2O (s)
+ NO2 (g) + Na2O (s) → 2 NaNO2 (s)
NO (g) + NO2 (g) → N2O (g) + O2 (g)
2 HNO2 (l) → N2O (g) + O2 (g) + H2O
ΔH = + 507 kJ
ΔH = - 427 kJ
ΔH = - 43 kJ
ΔH = + 34 kJ
(l)
12. Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
P4O10 (s) + 6 PCl5 (g)  10 Cl3PO (g)
A.)
B.)
C.)
D.)
P4 (s) + 6 Cl2 (g)  4 PCl3 (g)
P4 (s) + 5 O2 (g)  P4O10 (s)
PCl3 (g) + Cl2 (g)  PCl5 (g)
PCl3 (g) + ½ O2 (g)  Cl3PO (g)
ΔH
ΔH
ΔH
ΔH
=
=
=
=
-
1225.6 kJ
2967.3 kJ
84.2 kJ
285.7 kJ